1. Field of the Invention
This invention pertains to miniature hermetically sealed glass encapsulated ceramic capacitors and is more particularly directed to the formation of a metallurgical bond between the capacitor termination and the end cap.
2. Description of the Prior Art
The construction of a hermetically sealed, glass encapsulated ceramic capacitor has been discussed in an article entitled "Hermetic Glass Encapsulated Capacitors," by W. Love III and M. Rosenberg, published in the Proceedings of the IEEE 27th Electronic Components Conference, 1977, pages 387 to 390. U.S. Pat. No. 3,458,783 issued to the same M. Rosenberg discloses the hermetically sealed capacitor whose manufacture is discussed in the article just cited. According to this patent, a monolithic ceramic capacitor chip has opposed edge terminations coated with silver. These edges are located between and abutted by metallic surfaces of enlarged ends of leads for the capacitor. A glass sleeve surrounds the chip and the enlarged ends, and a glass-to-metal seal is effected between the sleeve and the enlarged ends, directly or through glass beads on such ends. By virtue of the heat seal, abutting copper and silver surfaces are caused to be bonded together. The contacting conductive surfaces in this construction are physically and conductively secured together. In effect, these abutting metal surfaces are fused together by a parent weld, that is, a weld formed of the two metals and without the use of flux or solder.
Many companies currently manufacture hermetically sealed, glass encapsulated ceramic capacitors using variations of the disclosures referenced above. These commercially available units for the most part contain silver terminated chip capacitors which are hermetically sealed in a glass sleeve between Dumet end caps at about 750.degree. C. These capacitors have an extremely high reliability, especially for use in applications subject to shock, vibration and temperature extremes. The hermetically sealed, glass-encapsulated package was specifically designed for these conditions.
The current failure rate, however, is somewhat higher than desirable and with certain sizes can run as high as 33 parts per million. Current quality control standards require a reliability exceeding this, namely a failure rate of less than 10 parts per million. The cause of the failures has been traced to a failure to achieve the required bond or stable contact between the silver termination and the Dumet end cap during the manufacturing process. If the bond is achieved, the failure rate will approach 0. Recent analyses of device failures indicate that after some temperature cycling, the contact between the silver terminations and Dumet end cap becomes intermittent and that this propensity is present ab initio in the failed units. Upon further study it is apparent from the pressures, temperatures and materials used, that no bond or weld is formed, contrary to the prior art statements. It would appear that the contact between the Dumet end cap and the silver termination of the chip capacitor is in effect, a simple pressure contact that is liable to become intermittent after some temperature cycling.
The present invention is designed to improve the contact between the leads of the glass package and the terminations on the capacitor within the package. To overcome the problems which have occurred, a metallurgical bond between the capacitor termination and the end cap would be a desirable solution. The present invention is directed to three methods for achieving such a metallurgical bond.
Another prior art patent which is relevant to the present invention is U.S. Pat. No. 4,117,589, owned by the assignee of this application. This patent discloses a method for establishing a metallurgical bond between the end termination of a resistor and the enlarged lead of its hermetically sealed glass package. In this prior patent, the emphasis is on the production of a package for a high reliability resistor. While the claims are not limited to resistors, the methods, materials and temperatures disclosed all indicate that the technique of this prior patent is not applicable for use with the hermetically sealed glass encapsulated capacitor.